Novel bonding structure for a hard disk drive suspension using anisotropic conductive film

ABSTRACT

A plurality of bonding structures and their forming methods for bonding a FPC to a bonding pad, in particular a bonding pad of a wireless suspension in a head gimbal assembly, using anisotropic conductive adhesive; such structures eliminate the spring-back force in typical anisotropic bonding to ensure durable bonding. At the same time, these structures also allow for reworkability under which the bonded parts can be separated easily.

BACKGROUND OF THE INVENTION

This invention generally relates to the field of disk drives, and moreparticularly to forming optimal structures for bonding in a head gimbalassembly using anisotropic conductive adhesive.

With the rapid progress of miniaturizing and thinning technology forelectronic devices, high-density inner wiring systems includingflex-print circuit (FPC) have become essential. At the same time,micro-connecting technology for the connection of FPC with otherelectronic parts, such as the traces on a magnetic head suspensionassembly, is indispensable.

Traditionally the FPC is capable of adopting ultrasonic bonding. Theconnecting terminals of the FPC are plated with gold; the flying leadsof the FPC are aligned with and pressed to the bonding pad on thesuspension with sufficient force to keep the alignment and atomicinterdiffusion of the flying leads and the underlying metallization,which process ensures the intimate contact between the two metalsurfaces. However, the pressing of the flying leads of the FPC entailscomplex processing, and ultrasonic bonding to different bonding pads isvery difficult to contact. Moreover, bonded parts cannot be separated inthe future to be reworked without damaging the FPC or the suspension.

Alternatively, FPC can be solder-bound using solder bumps produced by,for example, plating processes, for interconnections. However, thisprocess requires forming metal cores and solder bumps for soldering. Themetal cores incur extra expenses, and soldering has to be performed athigh temperatures typically over 180 degrees Celsius.

Furthermore, both ultrasonic bonding and soldering are becomingincreasingly expensive because of high cost of labor and parts of theFPC. There is therefore a need for a bonding method which achieves astable, reworkable connection without complicated processing.

SUMMARY OF THE INVENTION

The present invention features a novel structure and method for usinganisotropic conductive adhesive to bond parts in a head gimbal assembly(HGA) comprising the slider and the FPC.

It is an object of the present invention to overcome the complexities ofprior art approaches of ultrasonic bonding and soldering. This inventionwill alleviate the difficulty of one-time bonding in the case ofultrasonic bonding, and avoid high-temperature bonding required insoldering.

It is another objective of the present invention to reduce the bondingpad size and floating capacity.

Yet another objective of the present invention is to reduce the spacebetween bonding pads to accommodate the trend toward miniaturization ofthe disk drives and the head assemblies.

A further related objective of the invention is to improve capacity inthe bonding process. Reduced sizes of the bonding pads, reduced spacingbetween the bonding pads, and elimination of additional interconnectingcomponents will contribute to reduce parasitic capacitance. Reducedcapacitance will improve the rise and fall time of the electronicsignals, thus increase the data rate of the hard disk drive.

In one aspect, the invention relates to adding a conducting structurelodged between the two sections of an overcoat layer of a FPC to enablebonding between the FPC and a contact pad in a HGA using anisotropicconductive adhesive, such as anisotropic conductive film (ACF). Theconductive structure can be shaped as a ball and plated with gold, or itcan of other types of conductive materials. The overcoat layer mayoverlap a portion of the top surface of the conductive pad, or theovercoat layer may not touch the conductive pad at all. Alternatively,the conductive structure may be a filler comprising an electricallyconductive material completely filling the space between the twosections of the overcoat layer and above the conductive pad. In oneimplementation, the overcoat layer may comprise one section, or it maybe of ultra thinness of less than 10 μm.

In another aspect of the invention, a conductive layer of the FPC may bebound to the contact pad directly by anisotropic conductive adhesivematerial without an overcoat layer in between.

Other features and advantages of the present invention will becomeapparent from the following drawings and the detailed descriptionaccompanying the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a wireless suspension of a head gimbal assembly.

FIG. 2 is a top view of a FPC bound to the wireless suspension of FIG.1.

FIG. 3 is cross-sectional view of the structure of a conventional FPC.

FIG. 4 is a cross-sectional view of the structure of a wirelesssuspension bonding pad.

FIG. 5A is a cross-sectional view of the conventional FPC of FIG. 2positioned on top of the wireless suspension bonding pad of FIG. 4.

FIG. 5B is a cross-sectional view of the conventional FPC of FIG. 2bound to the wireless suspension bonding pad of FIG. 4 using anisotropicconductive adhesive.

FIG. 5C is a cross-sectional view, after reliability test, of aconventional FPC of FIG. 2 bound to the wireless suspension bonding padof FIG. 4 using anisotropic conductive adhesive.

FIG. 6 is a cross-sectional view of a novel bonding structure between aFPC and a wireless suspension using anisotropic conductive adhesive.

FIG. 7 is a cross-sectional view of a second novel bonding structure ofa FPC.

FIG. 8 is a cross-sectional view of a third novel bonding structurebetween a FPC and a wireless suspension using anisotropic conductiveadhesive.

FIG. 9 is a cross-sectional view of a fourth novel bonding structurebetween a FPC and a wireless suspension using anisotropic conductiveadhesive.

FIG. 10 is a cross-sectional view of a fifth novel bonding structure ofa FPC.

Like parts in different drawings are labeled with like numbers.

DESCRIPTION OF THE PREFERED EMBODIMENT

Referring to FIG. 1, this is a standard wireless suspensions. Trace 112is patterned on top of a flexture piece which runs from slider 120 tobonding pads 102, 104, 106, and 108, transporting electro-magneticsignals from slider 120. Base plate 100 supports bonding pads 102, 104,106, 108, to which a FPC is bonded for transmitting signals to elsewherein a hard disk drive, such as a circuit on the actuator arm. The numberof contact pads shown here is for illustrative purposes only, and therecould be more or fewer contact pads without deviating from the spirit ofthe invention.

Referring to FIG. 2, a FPC 200 is attached to contact pads 102, 104,106, 108 (not shown) in the circled area 210. Traditionally, FPC can bebound to contact pads using ultrasonic bonding or soldering. Withsoldering, additional solder bumps need to be incorporated As mentioned,both prior art bonding methods tend to be cost- and labor-intensive, andbonding using anisotropic conductive adhesive, such as anisotropicconductive film (ACF) CP 9252KS by Sony Corporation of Tokyo, Japan,presents a good alternative.

ACF bonding requires bonding temperature of 150 to 200 Celsius, and apressure environment of 20 to 40 kg per square centimeters. The bondingtime is about 10 to 20 seconds. The process involves cutting the ACFinto pieces of desirable size, tacking the pieces unto the surface to bebound, removing the release liner, and bonding under the conditions setout above. ACF bonding also offers the advantage of reworkability. Forexample, Sony CP9252KS can be reworked by dipping it in acetone for 2minutes, peeling the ACF, and following up with a Q-tip touch withacetone. ACF bonding also offers good bonding strength. For example,ultrasonic bonding typically offers a bonding strength of about 60 g,comparing with more 130 g for ACF bonding.

Despite the advantages offered by ACF bonding, difficulties remain forapplying ACF bonding to a head gimbal assembly. For example, FIG. 3shows a cross-sectional view of a conventional FPC structure. Aconventional FPC 200 usually comprises a base film 301, two sections 305and 309 of an overcoat layer, with an in-between conductive layer 303between base film 301 and the overcoat layer. Base film 302 is usuallymade of insulation material such as polyimide or other types of resinThe sections 305 and 309 of the overcoat layer is made of solder epoxy,photo sensitive solder resist materials, or polyimide film. Theconductive layer 303 is usually made of Cu or other similar materials.Between the sections 305 and 309 is the bonding pad surface 307, usuallywith a plating of Ni with thickness of about 4 μm and a plating of Auwith thickness of 1 μm.

FIG. 4 illustrates cross-sectional view of an assembly 400 comprising awireless suspension bonding pad, such as bonding pad 108 of FIG. 1.Assembly 400 comprises stainless steel base 401, on top of which is aninsulating layer 403. Insulating layer 403 can e made of polyimide orother types of insulating resin Bonding pad 108 is positioned on top oflayer 403, and it comprises, in a typical configuration, an electrode405 made of Cu, followed by a plating 407 of Ni, and finally a plating409 of gold at the outermost surface of bonding pad 108.

FIGS. 5A-5C illustrate some of the problems of using ACF to bond the FPC200 to the assembly 400. FIG. 5A shows that the FPC 200 is positioned ontop of assembly 400, with bottom surfaces of sections 305 and 309overlapping the two ends of bonding pad 108. When ACF film is heated andapplied to bond the two components using bonding tools and processingconditions as set forth above, a deformation 510 in the shape of abridge is formed to make contact between the FPC 200 and assembly 400,as shown in FIG. SB. Unfortunately, after reliability test, thisdeformation 510 tends to revert back to its original condition, causingan open circuit problem, as shown in FIG. 5C. Therefore, several novelbonding structures have been invented to solve this open circuitproblem.

Illustrated in FIG. 6 is a ball structure 610 which is placed betweenthe conductive layer 303 and the top surface of bonding pad 108. Theball structure 610 can be made of gold in one implementation, or it canbe made of other materials in other implementations of the invention.The ball structure 610 can be formed, in one implementation, with studbump bonding (SBB) flip chip method or gold ball bonding method commonlyknown in the art. The space surrounding ball structure 610, as well asspace 605 and 607, will be filled with melted/cured ACF used forbonding. The presence of structure 610 prevents the deformation of theFPC, and therefore eliminates the open circuit problem. Typically, for abase film of thickness 23 μm, the conductive layer is about 18 μm, andthe overcoat layer about 13 μm. Therefore, the ball structure, or bump610, has a height of approximately 13 μm. Circuit traces are labeled as601 and 602 in FIG. 6.

Alternatively, as illustrated in FIG. 7, the complete space formed bythe top surface of bonding pad 108 (not shown), the bottom surface ofconductive layer 303, and the right wall of overcoat section 305 andovercoat section 309 can be filled with filling materials 700. Thethickness of this filling 700 is about 13 μm, and it be made of a numberof conductive materials including Ni, Au, or a combination thereof. Inother implementations of the invention, the filling 700 can be thicker,thinner, to equal to the thickness of the overcoat layer, rangingbetween 10 to 38 μm. Using a solid filling 700 will achieve the sameobjective of eliminating the deformation bridge 510, and therebypreventing the open circuit problem. Note that adhesive layers used inthe manufacturing process of FPC 200 may still be present between thebase film 301 and conductive layer 303, and/or between conductive layer303 and overcoat sections 305 and 309.

Another implementation of the invention is the removal of one of the twoovercoat sections. In this configuration, as illustrated in FIG. 8, ballstructure 610 is still present, but the remaining section 805, theconductive layer 803 and the base film 801 are all of shorter lengththan their counterparts in a FIG. 6. This approach reduces the amount ofmanufacturing materials required. Melted/cured ACF fills spacesurrounding ball structure 610 and space 810.

FIG. 9 illustrates yet another implementation of the invention. In thisconfiguration, only one of the two sections of overcoat layer ispresent. The bottom surface of section 905 does not overlap the topsurface of bonding pad 108. Furthermore, this configuration does notrequire ball structure 610. At the same time conductive layer 903 bindsto the top surface of bonding pad 108 directly using ACF bonding, butdoes not overlap the top surface completely. Base film 901 extendsbeyond the length of bonding pad 108, but stops before reaching circuittrace 602. Eliminating the overcoat layer in a FPC will minimize theopen circuit problem; however, overcoat section 905 is needed to preventthe shunting problem around the complicated circuit pattern around thebonding pad. This contrasts with the right hand side of bonding pad 108,where conductive layer 903 does not touch trace 602 because of theabsence of an overcoat layer between it and trace 602. Therefore, thisconfiguration presents an optimal compromise between the elimination ofthe bridge deformation in a FPC inherent in ACF bonding, and theprevention of shunting problem around a bonding pad's complicatedcircuitry.

FIG. 10 illustrates another novel structure of FPC using ACF bonding.Because, as mentioned above, that it is impossible to eliminate theovercoat layer completely, one solution is to form an ultrathin overcoatlayer, such as presented in FIG. 10. Overcoat sections 1005 and 1010 areof less than 10 μm thick. They are think enough to prevent the shuntingproblem, but thin enough to prevent the formation of a deformationbridge in ACF bonding. Because sections 1005 and 1010 are thin, bondingsurface 1000 can bond directly to the top surface of a bonding padwithout causing a deformation in base film 301 and conductive layer 303.

The above embodiments of the invention are for illustrative purposesonly. Many widely different embodiments of the present invention may beadopted without departing from the spirit and scope of the invention.Those skilled in the art will recognize that the method and structuresof the present invention has many applications, and that the presentinvention is not limited to the specific embodiments described in thespecification and should cover conventionally known variations andmodifications to the system components described herein.

1-26. (canceled)
 27. A head gimbal assembly (HGA) circuit structureattached to a bonding pad on a suspension of a head gimbal assembly foruse in a hard disk drive using anisotropic conductive adhesive,comprising: a base film; a conductive layer situated below the basefilm, a part of said conductive layer attached to the bonding pad usingsaid anisotropic conductive adhesive; and an overcoat layer situatedbelow a portion of the conductive layer, a bottom surface of saidovercoat layer not overlapping a top surface of the bonding pad.
 28. TheHGA circuit structure of claim 27, further comprising a conductive ballpositioned above the bonding pad forming an electric conduit between theconductive layer and the bonding pad.
 29. The HGA circuit of claim 28,wherein the conductive ball comprises gold.
 30. The HGA circuitstructure of claim 27, wherein the anisotropic conductive adhesivecomprises anisotropic conductive film.
 31. The HGA circuit structure ofclaim 27, wherein a portion of said conductive layer is bonded to thetop surface of the bonding pad directly using said anisotropicconductive adhesive.
 32. A method for bonding a flex-print circuit to asuspension in a head gimbal assembly, comprising the steps of: Forming aconductive structure between a bonding pad and a conductive layer of theflex-print circuit; and Bonding the conductive layer to the bonding padvia the conductive structure using anisotropic conductive adhesive. 33.The method of claim 32, wherein the anisotropic conductive adhesivecomprises anisotropic conductive film.
 34. The method of claim 32,wherein the conductive structure comprises a gold ball.
 35. The methodof claim 32, wherein the conductive structure comprises a solidconductive material filling.
 36. The method of claim 34, wherein thegold ball is formed using stud bump bonding (SBB).
 37. The HGA circuitstructure of claim 27, wherein the overcoat layer comprises two sectionsseparated by a plating of conductive material, each of said two sectionsoverlapping an end of a top surface of the bonding pad.
 38. A HGAcircuit structure according to claim 37 wherein the filling is thinnerthan the overcoat layer.
 39. A HGA circuit structure according to claim37 wherein the base film extends beyond a first end of the conductivelayer and wherein the overcoat layer does not overlap any portion of thebonding pad when the bonding device is attached to the bonding pad.